1. Field of the Invention
The present invention relates to a structure of a peripheral transistor in a nonvolatile semiconductor memory device.
2. Description of the Related Art
In nonvolatile semiconductor memory devices such as NAND type flash memories, an FN (Fowler-Nordheim) tunnel current is applied to a gate insulating film so that a floating gate electrode (charge storage layer) is filled with electric charges at the time of writing. At this time, a positive voltage of 15 to 40V is applied to the control gate electrode.
When a capacitance ratio (coupling ratio) of the gate insulating film and an inter-electrode insulating film (dielectric film) of a memory cell is large, a high electric field is generated efficiently on the gate insulating film. As a result, a writing characteristic is improved.
Therefore, a technique using a high-permittivity (High-k) material for an inter-electrode insulating film has been proposed as a technique for improving the coupling ratio (for example, see Jpn. Pat. Appln. KOKAI Publication Nos. 2005-277171 and 2006-186073).
The nonvolatile semiconductor memory devices include a memory cell and a peripheral transistor. Both of them are formed by common processes as much as possible in order to reduce the process cost.
For this reason, the peripheral transistor has a structure similar to a memory cell, namely, a stack structure of lower electrode/inter-electrode insulating film/upper electrode. The lower electrode and the upper electrode are electrically connected via an opening provided on the inter-electrode insulating film.
In this case, the high-permittivity material which is adopted for improving the coupling ratio of the memory cell is used also for the inter-electrode insulating film of the peripheral transistor.
However, the high-permittivity material which is used for the inter-electrode insulating film of the peripheral transistor causes a problem that an off-leak current of the peripheral transistor increases and a field isolation breakdown voltage is lowered.
For example, when the high-permittivity material includes a metal oxide film or a raw material gas which is used for depositing the metal oxide film includes carbon atoms, the carbon atoms diffuse on a surface of a semiconductor substrate and a bottom portion of a field isolation insulating film.
When the high-permittivity material includes a nitride film obtained by plasma nitridation, nitrogen radicals at the time of deposition similarly diffuse on the surface of the semiconductor substrate and the bottom portion of the field isolation insulating film. As a result, a parasitic transistor is formed, which causes a problem that an off-leak current of the peripheral transistor increases and the field isolation breakdown voltage decreases.
Also after the inter-electrode insulating film is deposited, the carbon atoms and/or the nitrogen atoms in the inter-electrode insulating film diffuse. As a result, fixed electric charges are generated in an element area, and the performance of the peripheral transistor is possibly deteriorated.
If the high-permittivity material is intended to be used for the inter-electrode insulating film of the memory cell, the problems, such as the increase in the off-leak current and the reduction in the field isolation breakdown voltage, should be eliminated, in view of process integration, without increasing the number of steps of a manufacturing process (process cost).